1. Field of the Invention
This invention relates to an underwater cable arrangement capable of being used, for example, in seismic testing, as well as to various internal and external devices capable of being used with an underwater cable.
2. Description of the Related Art
In marine seismic exploration, an underwater cable, commonly referred to as a streamer cable, is towed through the water by a vessel such as a surface ship. An array of hydrophones is mounted within the cable, and an acoustic source or gun is fired to force an impulse of compressed air into the water, creating a bubble. The collapse of the bubble generates acoustic pulses that radiate through the water and into the earth. Reflections of the pulses off geologic structures are picked up by the hydrophones, and data representing the detected reflections are transmitted to the towing vessel. By analyzing the reflections, it is possible to discover subsea formations of oil or gas.
Various devices requiring electric power for their operation are mounted on the exterior of the underwater cable. For example, in order to accurately control the depth of the cable as it is being towed through the water, depth control mechanisms, commonly referred to as "cable-leveling birds", are attached to the cable at intervals along its length. The depth control mechanisms are equipped with adjustable diving planes, the angles of attack of which can be varied by motors in the depth control mechanisms so as to maintain the cable at a desired depth. Another type of external device frequently mounted on an underwater cable is an acoustic ranging device, which together with other acoustic devices is used to determine the locations with respect to the towing vessel of various points along the underwater cable to permit the exact shape of the cable during towing to be determined. Yet another type of commonly used external device is a magnetic heading sensor (also referred to as a magnetic compass), which determines the heading of the underwater cable at the point where the heading sensor is attached to the underwater cable.
It is possible to connect such external devices with a source of power aboard the towing vessel by means of wires passing through the skin of the cable, but the necessity of forming holes in the skin for the wires can lead to leakage of water into the interior of the cable, which is highly undesirable. Therefore, the external devices are usually self-powered by their own internal batteries. However, batteries have a number of serious drawbacks. First, the batteries have a limited life span and must be replaced every one to three months. Not only is the replacement of the batteries time-consuming, there is also the risk of exposing electric circuitry within the external device to sea water during replacement. In addition, lithium primary batteries, which are used because of their longer operating life compared to other batteries, are expensive, and spent batteries must be retained for proper disposal and not randomly discarded. Furthermore, each time the cable is reeled in to replace batteries in the external devices, it is subjected to stresses which frequently result in damage to the cable. Therefore, it is preferable to reel in the cable as infrequently as possible.
Another shortcoming of conventional external devices used with underwater cables is that the entire device must be removed from the cable before the cable is rolled up onto a reel on the deck of a towing vessel in order to prevent damage to the external devices and the cable. When the cable is to be redeployed, the external devices must be reattached to it. In rough seas, it can be extremely difficult and often dangerous for workers to remove the external devices from or reattach them to the cable on the deck of the towing vessel. Furthermore, onboard storage space, often a precious commodity, must be provided for the external devices on board the towing vessel. In addition, the reeling in or paying out of the underwater cable must be stopped during removal or reattachment of the external devices, so deployment and retrieval of the cable are time consuming.